<form onsubmit="event.preventDefault();" role="search">
  <label for="search">Search for stuff</label>
  <input id="search" type="search" placeholder="Search..." autofocus required />
  <button type="submit">Go</button>    
</form>

//Vars 
:root {
  --rad: .7rem;
  --dur: .3s;
  --color-dark: #2f2f2f;
  --color-light: #fff;
  --color-brand: #57bd84;
  --font-fam: 'Lato', sans-serif;
  --height: 5rem;
  --btn-width: 6rem;
  --bez: cubic-bezier(0, 0, 0.43, 1.49);
}

// Setup
body {background: var(--color-dark); display: flex; align-items: center; justify-content: center; min-height: 100vh }
html { box-sizing: border-box; height: 100%; font-size: 10px; } *, *::before, *::after { box-sizing: inherit; }

// Main styles
form {
  position: relative;
  width: 30rem;
  background: var(--color-brand);
  border-radius: var(--rad);
}
input, button {
  height: var(--height);
  font-family: var(--font-fam);
  border: 0;
  color: var(--color-dark);
  font-size: 1.8rem;
}
input[type="search"] {
  outline: 0; // <-- shold probably remove this for better accessibility, adding for demo aesthetics for now.
  width: 100%;
  background: var(--color-light);
  padding: 0 1.6rem;
  border-radius: var(--rad);
  appearance: none; //for iOS input[type="search"] roundedness issue. border-radius alone doesn't work
  transition: all var(--dur) var(--bez);
  transition-property: width, border-radius;
  z-index: 1;
  position: relative;
}
button {
  display: none; // prevent being able to tab to it
  position: absolute;
  top: 0;
  right: 0;
  width: var(--btn-width);
  font-weight: bold;
  background: var(--color-brand);
  border-radius: 0 var(--rad) var(--rad) 0;
}
input:not(:placeholder-shown) {
  border-radius: var(--rad) 0 0 var(--rad);
  width: calc(100% - var(--btn-width));
  + button {
    display: block;
  }
}
label {
  position: absolute;
  clip: rect(1px, 1px, 1px, 1px);
  padding: 0;
  border: 0;
  height: 1px;
  width: 1px;
  overflow: hidden;
}